text: Per- and polyfluoroalkyl substances are defined as a group of synthetic organofluorine chemical compounds that have multiple fluorine atoms attached to an alkyl chain. An early definition, from 2011, required that they contain at least one perfluoroalkyl moiety, –CF–. Beginning in 2021, the Organisation for Economic Co-operation and Development (OECD) expanded its terminology, stating that "PFASs are defined as fluorinated substances that contain at least one fully fluorinated methyl or methylene carbon atom (without any H/Cl/Br/I atom attached to it), i.e., with a few noted exceptions, any chemical with at least a perfluorinated methyl group (–CF) or a perfluorinated methylene group (–CF–) is a PFAS."
  According to the OECD, at least 4,730 distinct PFASs that contain at least three perfluorinated carbon atoms are known. The United States Environmental Protection Agency's (EPA) toxicity database, DSSTox, lists 14,735 unique PFAS chemical compounds, while PubChem lists more than 6 million.